The processor within a PC has evolved over the years, and with the addition of more processing power comes more heat in the case — and the need for more advanced cooling methods. This is a topic that regularly appears on the CompTIA A+ certification exams and is currently listed beneath objective 1.6 of the 220-801 exam. There are a few different types of CPU cooling methods, but the most important can be grouped into the categories of heat sinks, thermal paste and liquid-based methods.
We will take a look at both methods in the following guide to cooling — extracted from the CompTIA A+ Complete Deluxe Study Guide, Second Edition, on which I am a co-author — and look at what you need to know to pass this certification exam. You need to appreciate that heat is an enemy to the PC, and if any recently produced processor isn’t actively cooled all the time, it will generate enough heat to burn itself up in a short period of time and cause serious damage.
Most of the parts inside computers are cooled by air moving through the case and the CPU is no exception. Because of the large amount of heat produced, however, the CPU has to have the largest surface area (proportionally speaking) exposed to the moving air in the case. Therefore, the heat sinks on the CPU are the largest of any inside the computer.
A heat sink can be either active or passive. A passive heat sink sits close to the CPU and wicks away the heat into the air while an active heat sink has a fan that pulls the hot air away from the CPU. This fan often blows air down through the body of the heat sink to force the heat into the ambient internal air where it can join the airflow circuit for removal from the case. However, in some cases, you might find that the heat sink extends up farther, using radiator-type fins, and the fan is placed at a right angle and to the side of the heat sink. This design moves the heat away from the heat sink immediately, instead of pushing the air down through the heat sink.
There are CPU fans that have an adjustable rheostat allowing you to dial in as little or as much airflow as you need. These aid with noise reduction but can potentially lead to accidental overheating. The highest-performing CPU coolers use copper plates in direct contact with the CPU. They also use high-speed and high-CFM (cubic feet per minute) cooling fans to dissipate the heat produced by the processor.
Most newer CPU heat sinks use tubing to transfer heat away from the CPU — with any cooling system, the more surface area exposed to the cooling method, the better the cooling. The heat pipes can also be used to transfer heat to a location away from the heat source before cooling; this is especially useful in small form factor cases and laptops, where open space is limited.
With advanced heat sinks and other CPU cooling methods, it is important to improve the thermal transfer efficiency as much as possible. To that end, cooling engineers came up with a compound that helps to bridge the extremely small gaps between the CPU and the heat sink, which avoids superheated pockets of air that can lead to focal damage of the CPU. This product is known as thermal paste, thermal grease, thermal transfer compound or simply thermal compound and can be bought in small tubes (often resembling a syringe). Single-use tubes are also available and alleviate the guesswork involved with how much you should apply.
A small amount of the compound is applied as a bead in the center of the heat sink (not on the CPU). Some CPUs even have a raised area directly over where the silicon die is within the packaging, resulting in a smaller contact area between the components. Only a small amount is applied since the pressure of attaching the heat sink to the CPU will spread the compound across the entire surface in a very thin layer. Some new heat sinks have a patch of thermal compound preapplied, and you do not need to add more.
Liquid cooling is a technology in which a special water block is used to conduct heat away from the processor (as well as from the chipset). Water is circulated through the block to a radiator, where it is cooled. The theory is that you can achieve better cooling performance through the use of liquid cooling. For the most part, this is true. With traditional cooling methods (which use air and water), however, the lowest temperature you can achieve is room temperature. Plus with liquid cooling, the pump is submerged in the coolant (generally speaking), so as it works, it produces heat, which adds to the overall liquid temperature.
The main benefit of liquid cooling is silence. There is only one fan needed — the fan on the radiator, which cools the water — so a liquid-cooled system can run extremely quietly. While more efficient than air cooling and much quieter, most liquid-cooling systems are more expensive than supplemental fan sets, and require less familiar components, such as reservoir, pump, water block(s), hose and radiator.
The relative complexity of installing liquid cooling systems, coupled with the perceived danger of liquids in close proximity to electronics, leads most computer owners to consider liquid cooling a novelty or a liability. The primary market for liquid cooling is the high-performance niche that engages in overclocking to some degree. However, developments in active air cooling, including extensive piping of heat away from the body of the heat sink, have kept advanced cooling methods out of the forefront. Nevertheless, advances in fluids with safer electrolytic properties and even viscosities keep liquid cooling viable.
Heat pipes are closed systems that employ some form of tubing filled with a liquid suitable for the applicable temperature range. Pure physics are used with this technology to achieve cooling to ambient temperatures; no outside mechanism is used. One end of the heat pipe is heated by the component being cooled. This causes the liquid at the heated end to evaporate and increase the relative pressure at that end of the heat pipe with respect to the cooler end. This pressure imbalance causes the heated vapor to equalize the pressure by migrating to the cooler end, where the vapor condenses and releases its heat, warming the nonheated end of the pipe. The cooler environment surrounding this end transfers the heat away from the pipe by convection. The condensed liquid drifts to the pipe’s walls and is drawn back to the heated end of the heat pipe by gravity or by a wicking material or texture that lines the inside of the pipe. Once the liquid returns, the process repeats.
Peltier Cooling Devices
Water- and air-cooling devices are extremely effective by themselves, but they are more effective when used with a device known as a Peltier cooling element. These devices, also known as thermoelectric coolers (TECs), facilitate the transfer of heat from one side of the element, made of one material, to the other side, made of a different material. Thus, they have a hot side and a cold side. The cold side should always be against the CPU surface, and optimally, the hot side should be mated with a heat sink or water block for heat dissipation. Consequently, TECs are not meant to replace air-cooling mechanisms but to complement them.
One of the downsides to TECs is the likelihood of condensation because of the sub-ambient temperatures these devices produce. Closed-cell foams can be used to guard against damage from condensation.
With phase-change cooling, the cooling effect from the change of a liquid to a gas is used to cool the inside of a PC. It is a very expensive method of cooling, but it does work. Most often, external air-conditioner-like pumps, coils and evaporators cool the coolant, which is sent, ice cold, to the heat sink blocks on the processor and chipset. Think of it as a water-cooling system that chills the water below room temperature. Unfortunately, this is easily the noisiest cooling method in this discussion. Its results cannot be ignored, however; it is possible to get CPU temps in the range of –4°F (–20°C). Normal CPU temperatures hover between 104°F and 122°F (40°C and 50°C).
The major drawback to this method is that in higher-humidity conditions, condensation can be a problem. The moisture from the air condenses on the heat sink and can run off onto and under the processor, thus shorting out the electronics. Designers of phase-change cooling systems offer solutions to help ensure this isn’t a problem. Products in the form of foam; silicone adhesive; and greaseless, non-curing adhesives are available to seal the surface and perimeter of the processor. Additionally, manufacturers sell gaskets and shims that correspond to specific processors, all designed to protect your delicate and expensive components from damage.
Summing it Up
The three main methods of cooling to know for the CompTIA A+ certification exam are heat sinks, thermal paste, and liquid-based. Know that heat sinks can be either passive or active, based on whether or not there is a fan associated with them. Know that thermal paste works by removing air gaps, and issues with liquid cooled machines include cost and problems with hoses or fittings, the pump, or the coolant.